1. Field of the Invention
The invention concerns the simulation of an inflatable thermal blanket for testing an air source used to provide thermally-controlled, pressurized air to the blanket. More particularly, the invention concerns an inflatable thermal blanket simulator that includes an airflow resistor. When fitted over the end of an air hose, the simulator presents the apparent airflow resistance of an inflatable thermal blanket to an air source that is coupled to the other end of the air hose.
2. Description of the Related Art
Inflatable thermal blankets that operate by convecting temperature-controlled air are well-established clinical tools that are used, for example, to treat hypothermia. Such blankets have been produced and sold by AUGUSTINE MEDICAL, INC., the assignee of this application, and are gaining a gathering acceptance as evidenced by increases in sales every year since their introduction in 1989 by the assignee. These blankets are described in detail in, for example, U.S. Pat. Nos. 4,572,188; 5,300,101; 5,300,102; 5,324,390; and 5,405,371, among others. All of these patents are assigned to AUGUSTINE MEDICAL, INC., and are incorporated into this application by this reference.
Air sources for inflatable thermal blankets are well known. One is described, for example, in U.S. patent application Ser. No. 08/525,407, filed Sep. 8, 1995, and assigned to AUGUSTINE MEDICAL, INC., the assignee of this patent application. Typical air sources are represented, for example, by the Bair Huger.RTM. Model 500/OR and Model 505 products, both available from AUGUSTINE MEDICAL, INC., the assignee of this application.
Commercially available air sources used to provide temperature-controlled pressurized air to inflatable thermal blankets include a heater or cooler and a blower unit which operate to provide a steady stream of temperature-conditioned air at a given flow rate. The temperature-conditioned air is ducted from the air source to an inflatable thermal blanket which disperses the temperature-controlled air around a patient in order to raise or lower the core body temperature of the patient. The temperature of the air which reaches the inflatable thermal blanket is a function of several factors including, but not limited to: 1.) the heating or cooling capacity of the temperature-conditioning unit; 2.) the blower capacity; 3.) the length and thermal conductivity of the duct (typically, an air hose) between the air source and the inflatable thermal blanket; and, 4.) the airflow resistance of the inflatable thermal blanket.
The thermal characteristic of an airflow leaving an air source is generally controlled by continuously sensing the temperature of the airflow and adjusting power provided to the heating or cooling unit to maintain the temperature at a constant setting. The temperature of the airflow at the end of the air hose that couples to the inflatable thermal blanket (the "distal end" of the air hose), however, depends greatly on the time that the air remains within the air hose between the air source and the inflatable thermal blanket. This time is generally referred to as "residence" time. The most important variable that affects residence time is the airflow resistance imposed by the inflatable thermal blanket.
As is known, various inflatable thermal blanket configurations are available, each configuration designed to accomplish a particular clinical or therapeutic purpose, and each presenting its own airflow resistance, which is likely to be different than the airflow resistance presented by a different, or no, inflatable thermal blanket.
None of the currently available air sources are designed to sense either the external resistance to which they are attached or the temperature at the distal end of an air hose. Therefore, calibration and verification of the airflow temperature available at the distal end of the air hose, which is the airflow provided to the inflatable thermal blanket, should be performed with the air source operating into the same resistive load as that which is presented by one or more typical inflatable thermal blankets. Calibration or verification could, of course, be performed by connecting an inflatable thermal blanket at the distal end of the air hose and inserting a temperature measuring device into the airflow; however, this method requires a large space, the sacrifice of an inflatable thermal blanket, and the use of external equipment. This method also suffers from the inability to control uniformly "fin effect" losses which occur through the temperature measuring device.
Accordingly, a need exists for provision of means useful for measurement of the temperature of an airflow delivered to an inflatable thermal blanket. Preferably such a means should present a test condition approximating the actual airflow resistance of an inflatable thermal blanket.